Annual revenue attributable to tenant.

No definition available.

Name:	alx_AnnualRevenueAttributableToTenant
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Asset sale commission threshold.

+ References

No definition available.

+ Details

Name:	alx_AssetSaleCommissionThreshold
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of development costs that the related party will receive as per the Development agreement.

+ References

No definition available.

+ Details

Name:	alx_DevelopmentFeePercentage
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of rent payable to related party for leasing services for the eleventh through twentieth years of the lease.

+ References

No definition available.

+ Details

Name:	alx_LeaseFeePercentageOfRentElevenToTwentyYears
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of rent payable to related party for leasing services for the first ten years of the lease.

+ References

No definition available.

+ Details

Name:	alx_LeaseFeePercentageOfRentOneToTenYears
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of rent payable to related party for leasing services for the twenty-first through thirtieth years of the lease.

+ References

No definition available.

+ Details

Name:	alx_LeaseFeePercentageOfRentTwentyFirstToThirtyYears
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of management fee paid to related party to manage corporate activities.

+ References

No definition available.

+ Details

Name:	alx_ManagementFeeAgreementValue
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage of gross proceeds over $50,000,000 from the sale of any assets, payable to related party.

+ References

No definition available.

+ Details

Name:	alx_PercentageCommissionsOnSaleOfAssetsOverFiftyMillion
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of gross proceeds under $50,000,000 from the sale of any assets, payable to related party.

+ References

No definition available.

+ Details

Name:	alx_PercentageCommissionsOnSaleOfAssetsUnderFiftyMillion
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of increase in leasing services payable to related party if a real estate broker is used

+ References

No definition available.

+ Details

Name:	alx_PercentageIncreaseLeaseFeeIfBrokerUsed
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount per square foot paid to related party as property management fees.

+ References

No definition available.

+ Details

Name:	alx_PropertyManagementFeeAgreementPricePerSquareFoot
Namespace Prefix:	alx_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- Definition

Annual amount paid to related party as property management fees.

+ References

No definition available.

+ Details

Name:	alx_PropertyManagementFeeAgreementValue
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Annual increase in the amount paid to related party as property management fees.

+ References

No definition available.

+ Details

Name:	alx_PropertyManagementFeeEscalationPercentagePerAnnum
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of lease cost recognized by lessee for lease contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 55
-Paragraph 53
-URI http://asc.fasb.org/extlink&oid=77888399&loc=SL77918982-209971

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918686-209980

+ Details

Name:	us-gaap_LeaseCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of commissions expense incurred because the lessor of real estate obtained a lessee for a rental property through a real estate agent.

+ References

No definition available.

+ Details

Name:	us-gaap_LeasingCommissionsExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage charged for managing real estate properties.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyManagementFeePercentFee
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Receivables to be collected from (obligations owed to) related parties, net as of the balance sheet date where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_RelatedPartyTransactionDueFromToRelatedParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_RelatedPartyTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	alx_RealEstatePropertyOccupantAxis=alx_ToysRUsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorCustomersAxis=alx_BloombergMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=alx_LeaseAmendmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=alx_ThirdPartyBrokerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_RegoPark2PropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=us-gaap_RetailSiteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_AffiliatedEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=alx_LeasingFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=alx_PropertyManagementFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_LexingtonAvenuePropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=alx_OfficeAndRetailSpaceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=alx_CommonAreaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=alx_DevelopmentFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_AlexanderApartmentTowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=alx_ManagementPropertyManagementCleaningAndSecurityFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Party Transactions - Additional Information (Detail) ft² in Thousands, $ in Thousands	1 Months Ended	9 Months Ended
	Jan. 31, 2016 USD ($)	Sep. 30, 2017 USD ($) ft² $ / ft²	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)
Toys R Us [Member]
Related Party Transaction [Line Items]
Lease arrangements, unamortized assets		$ 1,617
Bloomberg [Member] \| Lease Amendment [Member]
Related Party Transaction [Line Items]
Leasing commission	$ 8,916
Bloomberg [Member] \| Lease Amendment [Member] \| Third Party Broker [Member]
Related Party Transaction [Line Items]
Leasing commission	7,200
Rego Park II [Member] \| Retail Space [Member] \| Toys R Us [Member]
Related Party Transaction [Line Items]
Area of property \| ft²		47
Annual revenue from leased property		$ 2,700
Vornado [Member]
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners		32.40%
Management fee agreement value		$ 2,800
Vornado [Member] \| Toys R Us [Member]
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage by parent		32.50%
Vornado [Member] \| Bloomberg [Member] \| Lease Amendment [Member]
Related Party Transaction [Line Items]
Leasing commission	$ 1,716
Vornado [Member] \| Leasing Fees [Member]
Related Party Transaction [Line Items]
Lease fee percentage of rent one to ten years		3.00%
Lease fee percentage of rent eleven to twenty years		2.00%
Lease fee percentage of rent twenty first to thirty years		1.00%
Percentage increase lease fee if broker used		1.00%
Asset sale commission threshold		$ 50,000
Percentage commissions on sale of assets under fifty million		3.00%
Percentage commissions on sale of assets over fifty million		1.00%
Amount Due to / (Fees paid to) related party		$ 2	$ 415
Vornado [Member] \| Property Management Fees and Payments for Cleaning and Security Services [Member] \| Rego Park II [Member] \| Retail Space [Member]
Related Party Transaction [Line Items]
Property management fee agreement percentage of income		2.00%
Vornado [Member] \| Property Management Fees and Payments for Cleaning and Security Services [Member] \| 731 Lexington Avenue [Member] \| Office and Retail Space [Member]
Related Party Transaction [Line Items]
Property management fee agreement price per square foot \| $ / ft²		0.50
Vornado [Member] \| Property Management Fees and Payments for Cleaning and Security Services [Member] \| 731 Lexington Avenue [Member] \| Common Area [Member]
Related Party Transaction [Line Items]
Property management fee agreement value		$ 306
Property management fee escalation percentage per annum		3.00%
Vornado [Member] \| Development Fees [Member]
Related Party Transaction [Line Items]
Development fee as percentage of development costs		6.00%
Amount Due to / (Fees paid to) related party		$ 24	54
Vornado [Member] \| Development Fees [Member] \| Alexander Apartment Tower [Member]
Related Party Transaction [Line Items]
Amount Due to / (Fees paid to) related party				$ 5,784
Vornado [Member] \| Management, Property Management, Cleaning and Security Fees [Member]
Related Party Transaction [Line Items]
Amount Due to / (Fees paid to) related party		$ 507	$ 428